This invention generally relates to devices and methods for extracorporeally treating blood or blood fractions such as blood filtrate or plasma to selectively remove toxins therefrom.
By way of background, extensive efforts have been made to discover safe and effective methods for removing toxins from patients by extracorporeal treatment of their blood. These efforts have included many studies directed to methods for extracorporeal treatment of hepatic failure due to infection, cirrhosis, toxin damage or other causes. Many methods have been proposed with the goal of removing small molecular toxins, protein-bound molecules or larger molecules thought to be responsible for the coma and illness of hepatic failure. Thus far, evidence has been presented supporting adverse effects caused by non-protein bound small molecules such as ammonia, phenols, mercaptans, short chain fatty acids, aromatic amino-acids, neural inhibitors (GABA, glutamate), false neural transmitters (octopamine) and bile salts. Among these, phenols and mercaptans, along with bilirubin and bacterial endotoxins, also occur as strong protein-bound toxins and are thus more difficult to effectively remove from the blood. In addition, there are a variety of middle molecular weight toxins of liver failure having molecular weights of about 300 to about 10,000 which are difficult to effectively remove.
As to specific modes of treatment, those previously proposed and used have included blood perfusion over heterogeneous liver pieces or past membranes which contact hepatocytes. Also proposed and used have been hemoperfusion through columns of coated activated carbon or macroreticular resins, blood exchange, plasmapheresis with plasma replacement, plasmapheresis with plasma perfusion through bilirubin-binding and aromatic amino acid-binding sorbents, standard hemodialysis, standard hemodialysis with an amino acid dialysate and plasma exchange, high permeability hemodialysis, dialysis with charcoal-impregnated membranes, continuous hemofiltration, peritoneal dialysis, oral sorbents and many other therapies.
While some of these previously proposed treatments have produced neurological improvement in stage 2 or 3 coma and have aided hepatic regeneration after injury, they have not provided much clinical improvement in patients in stage 4 coma on respirators. Additionally, these diverse treatments each produce adverse effects on the patient, offsetting benefits. See, generally, Ash, S. R., Treatment of Acute Hepatic Failure With Encephalopathy: A Review, Int. J. of Artif. Organs, Vol. 14, pp. 191-195 (1991).
For example, although daily charcoal hemoperfusion has been shown to provide neurologic and physiologic improvement of patients with hepatic failure and coma, Winchester, J. F., Hemoperfusion, in Replacement of Renal Function by Dialysis (Maher, J. F., ed.), Dordrecht:Kluwer Academic Publishers, pp. 439-459, (1989), hemoperfusion nevertheless requires systemic anticoagulation and also depletes coagulation factors and platelets from the blood. Moreover, the relatively large sorbent granules used in hemoperfusion columns have limited surface area (about 1000-10,000 m.sup.2). Consequently, the available sorbent surface area is saturated within a few hours, clearance of bound chemicals rapidly diminishes, and a new column must be used.
Furthermore, clinical benefits of charcoal hemoperfusion may be offset by deleterious effects of bioincompatibility. In one instance, a controlled study of patients with fulminant hepatic failure, all treated with aggressive intensive care including intracerebral pressure monitoring, demonstrated that patients treated by hemoperfusion had a generally lower survival rates than those treated with aggressive intensive care alone. The only exception was noted in patients having fulminant hepatic failure due to hepatitis A or B, for whom there was reported a "trend toward improved survival" when treated with charcoal perfusion. O'Grady, J. G. et al., Controlled Trials of Charcoal Hemoperfusion and Prognostic Factors in Fulminant Hepatic Failure, Gastroenterology, Vol. 94, pp. 1186-92 (1988).
As mentioned, standard hemodialysis (i.e. dialysis of blood against only a dialysate solution) has also been studied as a possible treatment for hepatic failure. However, benefits of hemodialysis may be similarly obscured by removal of substances (e.g. urea) known not to be toxins of hepatic failure. Additionally, hemodialysis requires the use of large volumes of dialysate solution which limits the mobility and increases the complexity of the machines, or alternatively, it requires the provision of a sorbent column to "regenerate" the dialysate.
In light of this extensive background, there remain needs for improved devices and methods for the extracorporeal treatment of blood or of blood fractions to effectively remove toxins, including both soluble and protein-bound toxins. The present invention addresses these needs.